1. Field of the Invention
The present invention relates to a thermal transfer recording method and a thermal transfer printer, and, more particularly, to a thermal transfer recording method and a thermal transfer printer, in which after thermally transferring underlying ink onto a record sheet, recording ink is thermally transferred onto the record sheet, whereby a thermal transfer recording operation is carried out.
2. Description of the Related Art
In general, in conventional thermal transfer printers, a record sheet is supported forwardly of a platen, and a thermal head, formed of a plurality of heating elements, is carried by a carriage. When an ink ribbon and the record sheet are nipped between the thermal head and the platen, and the heating elements of the thermal head are selectively heated, based on recording data, while the thermal head reciprocates along the platen, the ink of the ink ribbon is thermally transferred onto the record sheet, whereby a desired character, or the like, is recorded on the record sheet. Such conventional thermal transfer printers provide high quality printing, has a low noise level, is inexpensive, and is easy to maintain, so that they are frequently used in output devices of, for example, computers or word processors.
Two types of such conventional thermal transfer printers are well known. In the first type, a recording operation is performed on a record sheet with a thermal fusible ink ribbon formed by applying thermal fusible ink onto a resin film, or a base formed of, for example, polyethylene terephtalate (PET). In the second type, a recording operation is performed on a record sheet with a thermal sublimable ink ribbon formed by applying sublimable ink to a base.
When a recording operation is performed with a thermal sublimable ink ribbon, the energy applied to the thermal head is controlled to control adjust the amount by which the thermal sublimable ink is sublimated, whereby the amount of ink transferred onto the record sheet is controlled. This results in adjustment of the density of the image to be recorded on the record sheet. Using, as record sheet, a special sheet subjected to surface treatment, a high-quality and full color image comparable with a silver salt photograph can be obtained.
In recent years, there has been an increasing demand for a thermal sublimation transfer recording method which can be easily carried out to record a high-quality and full color image not only on a sheet formed specially for thermal sublimation transfer recording, but also on generally used record sheets such as post cards and ordinary sheets. When thermal sublimation transfer recording is performed on postcards or ordinary sheets without subjecting them to surface treatment, the thermal sublimable ink does not get transferred onto the postcards or ordinary sheets, so that a high-quality recorded image cannot be obtained. Thermal fusion transfer recording using a thermal fusible ribbon can be performed on an ordinary sheet. However, when a high-quality and full color image needs to be obtained, thermal fusion transfer recording must be performed on a special sheet with a smooth surface, instead of on an ordinary sheet with a rough surface.
To overcome the above-described problem, prior to performing thermal sublimation transfer recording or thermal fusion transfer recording, the surfaces of postcards, ordinary sheets, or the like, are subjected to underlying ink transferring operations to smoothen the sheet surfaces and to make it easier to transfer it thereon.
More specifically, an underlying ink transferring ink ribbon is formed by applying transferring underlying ink onto a resin film, or a base (formed of, for example, PET). The resulting ink ribbon is set in a thermal transfer printer, such as a serial printer, and the ink of the resulting ink ribbon is thermally transferred onto postcards or the like. This results in the production of postcards with a surface which is in general suitable for thermal sublimation transfer recording or thermal fusible transfer recording. When thermal sublimation transfer recording is performed on, for example, postcards subjected to such surface treatment, a high quality image comparable to that produced on sheets formed specially for thermal sublimation transfer recording can be obtained.
However, in underlying ink transferring operations carried out using such a thermal transfer printer, since the amounts of energy to be applied to the heating elements, disposed in the direction of a record sheet line, of the thermal head are controlled such that they are all the same, the ink of the upper and lower edge portions of the underlying ink transferring ink ribbon are not sufficiently transferred.
In other words, since the ink of the underlying ink transferring ink ribbon is hard, it is particularly hard to transfer the ink at the edge portions. Therefore, after the ink has been transferred, the boundary portions between adjacent underlying ink layers in a direction perpendicular to the direction of a record sheet line peel off. This may result in a disrupted image or an image with white streaks appearing at locations corresponding to where peeling has occurred.
To overcome this problem, an underlying ink layer is formed on a next line such that edges of underlying ink layers, formed by an underlying ink transferring ink ribbon on different lines, overlap each other. When thermal sublimation transfer printing (carried out using a thermal sublimable ink ribbon) is performed one line at a time on a record sheet subjected to the underlying ink transferring operation, or when thermal fusion transfer printing (carried out using a thermal fusible ink ribbon) is performed one line at a time on the record sheet subjected to underlying ink transferring operations, a high-quality and full color image can be obtained.
However, in conventional thermal transfer recording methods and thermal transfer printers, the starting location of underlying ink transferring operations and the starting location of recording operations are in line with each other, so that when recording operations are carried out using thermal sublimable or thermal fusible ink on a record sheet, subjected to underlying ink layer operations using a underlying ink transferring ink ribbon, the boundaries between adjacent record layers in a direction perpendicular to the direction of a record sheet line are formed in line with the boundaries between adjacent underlying ink layers in a direction perpendicular to the direction of a record sheet line. To improve image quality, various operations have been performed on the boundary portions between adjacent record layers, formed by the recording ink ribbon, so that each boundary portion does not stand out. When the boundaries between adjacent record layers are formed in line with the boundaries between adjacent underlying ink layers formed above the adjacent underlying ink layers, the recording density at these locations are different from the recording density at the peripheral portions. Therefore, it is no use carrying out operations on the boundary portions to prevent them from standing out. Consequently, even when underlying ink transferring operations are performed, the image quality cannot be improved.
In addition, in conventional thermal transfer recording methods and thermal transfer printers, underlying ink layer operations are performed the same number of times regardless of the sheet type, so that underlying ink transferring operations cannot be carried out in accordance with the properties of a record sheet type. In addition, during recording operations, the ink of a recording ink ribbon cannot be properly transferred onto a record sheet subjected to underlying ink layer transferring operations.